CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME Parenthetical - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Plan and Restoration of Retirement Income Plan:
Amortization of deferred net loss, net of tax of $1.2, $1.1 and $1.1, respectively	$ 1.2	$ 1.1	$ 1.1
Net loss arising during the period, net of tax of ($1.7), ($2.6) and ($4.7), respectively	(1.7)	(2.6)	(4.7)
Postretirement plans:
Amortization of deferred net gain, net of tax of $0.0, $0.0 and $0.0, respectively	0.0	0.0	0.0
Net gain (loss) arising during the period, net of tax of ($0.8), ($0.1) and $0.7, respectively	(0.8)	(0.1)	0.7
Amortization of prior service credit, net of tax of ($0.6), ($0.6) and ($0.6), respectively	(0.6)	(0.6)	(0.6)
Pension Plans [Member]
Pension Plan and Restoration of Retirement Income Plan:
Settlement cost, net of tax of $0.7, $2.7 and $1.6, respectively	0.7	2.7	1.6
Other Postretirement Benefits Plan [Member]
Pension Plan and Restoration of Retirement Income Plan:
Settlement cost, net of tax of $0.7, $2.7 and $1.6, respectively	$ (0.1)	$ 0.0
Postretirement plans:
Curtailment cost, net of tax of ($0.1), $0.0 and $0.0, respectively			$ 0.0